It has been recognized for over 50 years that certain tumor cells have a high demand for amino acids, such as L-arginine and are killed under conditions of L-arginine depletion (Wheatley and Campbell, 2002). In human cells L-arginine is synthesized in three steps; first L-citrulline is synthesized from L-ornithine and carbamoyl phosphate by the enzyme ornithine transcarbamylase (OTC), argininosuccinate synthetase (ASS) converts L-citrulline and aspartate to argininosuccinate, followed by conversion of argininosuccinate to L-arginine and fumarate by argininosuccinate lyase (ASL). A large number of hepatocellular carcinomas (HCC), melanomas and, renal cell carcinomas (Ensor et al., 2002; Feun et al., 2007; Yoon et al., 2007) do not express ASS and thus are sensitive to L-arginine depletion. The molecular basis for the lack of ASS expression appears to be diverse and includes aberrant gene regulation. Whereas non-malignant cells enter into quiescence (Go) when depleted of L-arginine and thus remain viable for several weeks, tumor cells have cell cycle defects that lead to the re-initiation of DNA synthesis even though protein synthesis is inhibited, in turn resulting in major imbalances and rapid cell death (Shen et al., 2006; Scott et al., 2000). The selective toxicity of L-arginine depletion for HCC, melanoma and other ASS-deficient cancer cells has been extensively demonstrated in vitro, in xenograft animal models and in clinical trials (Ensor et al., 2002; Feun et al., 2007; Shen et al., 2006; Izzo et al., 2004). Recently Cheng et al. (2007) demonstrated that many HCC cells are also deficient in ornithine transcarbamylase expression and thus, they are also susceptible to enzymatic L-arginine depletion.
There is interest in the use of L-arginine hydrolytic enzymes for cancer therapy, especially the treatment of cancers such as hepatocarcinomas, melanomas and renal cell carcinomas, for example, which are common forms of cancer associated with high morbidity. Two L-arginine degrading enzymes have been used for cancer therapy: bacterial arginine deiminase and human arginases. Unfortunately, both of these enzymes display significant shortcomings that present major impediments to clinical use (immunogenicity, and low catalytic activity with very poor stability in serum, respectively). Thus, the therapeutic success of L-arginine depletion therapy will rely on addressing these shortcomings.
Another challenge in the treatment of many cancers is the ability of some cancers to evade the immune system. Some tumors, for example, do this through the immune checkpoint pathways, which are inhibitory pathways in the immune system that maintain self-tolerance by modulating immune response. These pathways can be dysregulated by tumors resulting in immune resistance. Some of these pathways, both agonists of prostimulatory receptors or antagonists of inhibitory signals, both of which result in amplification of antigen-specific T-cell responses, have become targets for cancer immunotherapy. Some exemplary receptors and ligands include cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), programmed cell death 1 (PD1), programmed cell death ligand 1 (PDL1), lymphocyte activation gene 3 (LAG3), B7-H3, B-7-H4, and T cell membrane protein 3 (TIM3) among others. (Pardoll, 2012).